The blocks are 12 1/2" and the strips are all 2 1/2" wide. This makes a finished block of 12" when sewn together. I made a long queen size, which is 4 blocks across and 6 blocks down. I added borders of 2 1/2 (red) 4 1/2 (white) and 6 1/2 (green). It was supposed to be for a full bed, but turned out large enough for a queen because of the extra wide borders. This way it will hang down about 12" on each side of a full bed.
It all depends on what size bed you are trying to fit. 3 x 4 is for a twin, 4 x 5 or 4 x 6 for a full/queen and 5 x 5 or 5 x 6 for a king size.
Have fun with it, that is the only reason to quilt, for the enjoyment you get making it and the enjoyment the person who receives it gets using it.